This invention relates to an apparatus and a method for compensating for warm-up drift produced during the start-up phase of a position control apparatus.
An apparatus for controlling the position is initially driven by a rotating servo motor which is initially at a low a temperature. As a result of the continuous rotation of the servo motor, the servo motor generates thermal energy. A characteristic of one element (for example, a transistor) of the apparatus is subject to change (caused by a warm-up drift) which can cause an error in the rotation speed. In the particular, an error which exceeds the maximum permissible limits is generated due to the warm-up drift caused by the temperature change.
In order to solve these problems, the prior art discloses that (1) an initial drift compensation value is set at zero, (2) when a difference between a specific destination position and the present position exists within the maximum permissible limits every time the motor moves, the previous setting value will be repeatedly used and (3) if a difference between a specific destination position and the present position exists beyond the maximum permissible limits, the drift compensation value should be re-established.
Because such a prior art method for compensating for warm-up drift does not establish a new compensating value if the difference between the destination position and the present position is within the maximum permissible limits, the fluctuations will be within the permissible limits but will vary over time due to the initial warm-up drift.
In order to achieve a more efficient operation, until the apparatus controlling position can be more stabilized, a supplementary operation is needed at the initial start-up phase. A typical prior art for suppressing drift in a servo motor is disclosed at Japanese Laid Open Number 1-205988.
A method for suppressing drift according to the invention of this Japanese application receives a weight effect from the position of a joint in a teaching mode, outputs a torque compensating the weight from a joint actuator to the joint, drives a servo motor and sets a weakness position to the present position of the joint as a target position thereby reducing a load in teaching and suppressing a drift generated from a mechanical transition and weight calculation error in figure arithmetic.
Accordingly, this Japanese invention does not resolve the above problem in that it continuously uses a previously established compensation value if the difference between the destination position and the present position is within the maximum permissible limits.
The object of the invention is to reduce the time to attain to a more efficient operation by accurately compensating for warm-up drift of a position control apparatus in a servo motor.
Another object of the invention is to reset the warm-up drift compensation value even when the difference between the destination position and the present position is within the maximum permissible limits.
Another object of the invention is to reset the warm-up drift compensation value by adding the previous compensation value to a given value in an integral controller every time the servo motor stops.
Therefore, in order to achieve the object, an apparatus according to the present invention reads specific value via a register in an integral controller of a position control apparatus via a register every time that the speed feedback signal is applied to the register, totals said values, transmits a speed feedback signal and a speed command signal output from the PID (Proportional Integral Differentiation) controller and controls the servo motor based on the result of the calculations. It sets an initial warm-up drift compensation value at zero and thereafter resets the warm-up drift compensation value whenever the servo motor comes to a stand still.
A method for compensating for warm-up drift according to the invention comprises the steps of setting an integral limiter so as not to limit the output of the integral controller, initiating a warm-up drift compensation value, setting the warm-up drift compensation value at zero in the initial stopped position of the servo motor, the step of resetting an integral limiter so as not to generate any vibrations in the initial stopping of the servo motor, detecting the present position and determining if there is a difference between it and the destination position, and when the difference is within a maximum permissible limit, controlling a position control apparatus with a reset warm-up drift compensation value, and, when the difference exceeds a maximum permissible limit, setting the integral limiter so as not to limit the output from the integral controller, resetting the warm-up drift compensation value and setting the integral limiter so as not to generate any initial vibration.